1. Field of the Invention
This invention relates to the recovery of power from gas of low heating value and more particularly to a gas turbine system adapted to utilize such gas to generate power.
2. Description of the Prior Art
One method for increasing the production of heavy crude oils of high viscosity from underground formations is the in-situ combustion process. In that process, air is injected at a high pressure through an injection well into the underground formation containing the heavy oil. The oil in the formation is ignited adjacent the injection well by any of several known procedures such as the procedure disclosed in U.S. Pat. No. 3,172,472 of F. M. Smith. Injection of air is continued after ignition to burn part of the oil in the formation and to increase the pressure in the formation adjacent the injection well and thereby drive oil in the formation toward a production well spaced from the injection well. A typical in-situ combustion process is described in U.S. Pat. No. 2,771,951 of Simm. The heat released by combustion of some of the oil in the formation heats the formation and oil whereby the viscosity of the oil is greatly reduced by the high temperature, cracking of the oil, and by solution in the oil of low molecular weight hydrocarbons formed by the cracking. The reduced viscosity and the pressure of the injected gases cause the oil to flow through the underground reservoir to a production well.
During in-situ combustion processes, the combustion front at which oil in the formation is burned does not move radially outwardly from the injection well at a uniform rate in all directions. Some of the injected air fingers through zones of high permeability in the formation toward a production well and combustion occurs at the boundaries of the fingers. There is usually a breakthrough of combustion products in the nature of a flue gas long before the production of oil by the in-situ process is completed. Volatile constituents in the oil, or formed by cracking of the oil, are entrained in the injected air of flue gases and carried by them to the production well. All of these factors contribute toward a nonuniformity in the composition and heating value of the gas produced.
The fluids produced at the production well are separated into liquid petroleum products which are delivered to storage or a delivery line and gaseous products. The gaseous products customarily have been vented to the atmosphere. The gaseous products hereinafter referred to as LHV gas, from in-situ combustion of heavy petroleum contain low concentrations of methane and C.sub.2 -C.sub.6 hydrocarbons, as well as nitrogen, carbon dioxide, sulfur compounds such as hydrogen sulfide, mercaptan and carbonyl sulfide, and in some instances a small amount of carbon monoxide. Those gaseous products constitute low heating value fuel capable of supplying a substantial part of the energy required to compress the air for injection into the subsurface formation at the injection well. The shortage of natural gas makes it important that the energy in the production from an in-situ combustion process be fully utilized. Moreover, tightening of laws relating to pollution of the atmosphere has placed stringent limitations on the amount of carbon monoxide, the sulfur compounds most frequently present in the gaseous products and hydrocarbons other than methane that may be discharged into the atmosphere.
U.S. Pat. No. 3,113,620 of Hemminger describes a single well in-situ combustion process in which a cavity filled with rubble is formed in a subsurface oil shale deposit by means of a nuclear explosion. An in-situ combustion process in the cavity is then conducted to remove oil from the rock, aid in draining the oil into a pool in the bottom of the cavity, and force the oil up the well to the surface. The gas produced with the oil in the in-situ combustion of oil shale contains a higher concentration of carbon monoxide than gas produced in a conventional in-situ combustion process in an oil reservoir. Because gas produced from oil shale has a high concentration of carbon monoxide and hydrogen as compared to the off-gas from in-situ combustion in petroleum reservoirs, the off-gas from oil shale may in some instances be burned directly in a flame combustor of a gas turbine used to drive an air compressor.
In a paper entitled "Power Generation from Shale Oil Process Off-Gas" by J. M. McCrank and G. R. Short presented at the IEEE-ASME Joint Power Generation Conference in Buffalo, N.Y., Sept. 19-22, 1976, the results of an engineering study of generation of power from the off-gas from the in-situ combustion of oil shale are described. Off-gas from the oil shale is burned in a flame-type combustion to produce hot products of combustion used to drive a gas turbine. As indicated above, the high concentration of carbon monoxide in the off-gas from the in-situ retorting of oil shale facilitates flame combustion but variations in the composition of the off-gas could make maintenance of a stable flame uncertain.
A paper entitled "Catalysts for Gas Turbine Combustors- Experimental Test Results" by S. M. DeCorso, S. Mumford, R. Carrubba and R. Heck, presented at the Mar. 21-25, 1976 meeting of The American Society of Mechanical Engineers describes the burning of a fuel described as a low heating value gas in a laboratory catalytic combustion chamber. The low heating value gas is further identified as synthetic coal gas heat having a heating value of 126 Btu/scf. The combustion was accomplished by passing in contact with the catalyst a mixture of the gas and air preheated to a temperature whereby catathermal combustion of the type described in U.S. Pat. No. 3,928,961 of Pfefferle occurs. In that process, the mixture of fuel and air near the catalyst surface is at a temperature at which thermal combustion occurs at a rate higher than the catalytic rate and the catalyst surface is above the instantaneous autoignition temperature of the fuel-air mixture.